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Industrial Crops and Products 43 (2013) 114– 118

Contents lists available at SciVerse ScienceDirect

Industrial Crops and Products

journa l h o me pag e: www.elsev ier .com/ locate / indcrop

old plasma-assisted paper recycling

arla Gaiolasa, Ana Paula Costaa, Manuel Santos Silvaa, Wim Thielemansb,c, Maria Emília Amarala,∗

Research Unit of Textile and Paper Materials, University of Beira Interior, 6201-001 Covilhã, PortugalSchool of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UKProcess and Environmental Research Division, Faculty of Engineering, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK

r t i c l e i n f o

rticle history:eceived 5 July 2012ccepted 10 July 2012

eywords:aper recyclinglasma treatmentPStatistical analysis

a b s t r a c t

The aim of this study is to modify the surface of a commercial paper, using the cold plasmatreatment in order to increase its hydrophilic character, thus minimizing the disintegration timeand/or the energy consumption, needed to recycle cellulose fibers and obtain a homogeneoussuspension. Plasma treatment was applied for different exposure times, under optimal experi-mental conditions of ground pressure and power. The treated paper samples were characterizedby contact angle measurements. The plasma treated samples were disintegrated using a seriesof different number of rotations (rpm) and the resulting fiber suspensions were used to preparelaboratory hand sheets using a conventional sheet forming. Before paper making, the morpho-

ontact angle logical characteristics of the fibers were evaluated by a MorFi analyser. The effect of plasmatreatment on the quality of recycling was evaluated measuring the first-order entropy of the sheet for-mation. These results show that for similar entropy values, disintegration time for the reference samplesis longer than for the treated samples. X-ray photoelectron spectroscopy (XPS) showed that the surfaceof the treated samples underwent strong oxidation, which is probably responsible for easy recycling of

the paper samples.

. Introduction

Cold plasma treatment is an environmentally safe process,hich offers many advantages, namely: (i) it is a solvent free pro-

ess (no chemical pollution); (ii) it can be applied as a continuousrocess and (iii) it can be carried out under different controlledtmospheres, thus inducing a wide variety of chemical changes toield materials with various properties (Cheng et al., 2006; Gaiolast al., 2008; Popescu et al., 2011). Plasma can be defined as aartially ionized gas, which generates energetic species present

n the discharge, such as electrons, ions, free radicals and pho-ons, which possess sufficiently high energies to modify only theppermost atomic layers of a material surface, without alteringulk characteristics (Abidi and Hequet, 2004; Yuan et al., 2004;hen et al., 2011). Abidi and Hequet (2004) referred to the threeifferent and independent processes related to plasma technol-gy, namely: (1) modification of surface structure of the material

tself, under the influence of glow discharge, (2) plasma poly-

erization and (3) grafting of molecules on the material surfacefter plasma activation. In the last decade, plasma has received

∗ Corresponding author at: Research Unit of Textile and Paper Materials, Univer-ity of Beira Interior, Rua Marquês d’Avila e Bolama, 6201-001 Covilhã, Portugal.el.: +351 275314740; fax: +351 275314740.

E-mail address: mecca@ubi.pt (M.E. Amaral).

926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.indcrop.2012.07.016

© 2012 Elsevier B.V. All rights reserved.

considerable attention in the realm of the surface modificationof various natural materials, such as wood (Carlsson and Ström,1991; Verreault et al., 1990; Westerlind et al., 1987), cellulosefibers from different origins (Popescu et al., 2011; Vander-Wielenand Ragauskas, 2004; Vander Wielen et al., 2006; Vaswani et al.,2005), paper restoration (Vohrer et al., 2001) and also in tex-tile fibers (Abidi and Hequet, 2004; Chen et al., 2011; Höcker,2002).

To the best of our knowledge, there are no studies on plasmatreatment to assist paper recycling. This process represents a sec-tion of the paper industry that is oriented toward re-use andsustainability, which implies the preservation of essential wood-land resources (Pèlach et al., 2003). The use of recovered paper forpapermaking requires the re-wetting and its reduction to a fibroussuspension, a process known as disintegration. At a non-integratedpaper industry, disintegration takes place inside pulper equipment.Savolainen et al. (1991) stated that a pulper consumes about 5–15%of the total energy used to obtain the final product. Therefore, aneco-friendly technique contributing to minimize this parametercan be very useful and interesting.

This article reports preliminary results obtained from the useof plasma discharge to increase the hydrophilic character of

a commercial paper. The treated and untreated samples werecharacterized by contact-angle measurements and XPS analyses,before being submitted to recycling operation, in order to pro-duce a homogeneous fibrous suspension. The produced fibers were

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After plasma treatment, the surface shifted from hydrophobic(water contact angle of 110◦) to hydrophilic character (water con-tact angle of 28◦). In fact, the work of adhesion of a droplet of water

20

40

60

80

100

120

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(θ°

)

C. Gaiolas et al. / Industrial Cro

haracterized in terms of morphological properties using a MorFiber analyzer.

. Materials and methods

.1. Raw material and disintegration

A commercial paper (reference paper) with a basis weight of0 g m−2 was used as raw material. The reference samples andhose treated with plasma were disintegrated in a laboratoryisintegrator (ISO 5263-1) varying the number of rotations, asollows: 2500, 5000, 7500, 10,000 and 20,000 rpm. This equip-

ent consists of an integrator motor, an impeller, a removableup impact device and a pre-selection to adjust the stirring speedAmaral et al., 2000). These resulting suspensions were used toroduce laboratory hand sheets (with a basis weight of 60 g m−2)sing conventional sheet forming process, according ISO 5269-1tandard.

.2. Plasma treatment

The used radio frequency plasma generator was a EUROPLASMApparatus equipped with a microcontroller, a vacuum system and

2.54 GHz microwave generator. The treatment power was 200 Wt a constant pressure of 700 mTorr, as already established in ourrevious studies (Gaiolas et al., 2008, 2009). The treatment timearied from 5, 10, 15, 30, 60, 120, 180 and 300 s, in the presence ofir. The vacuum level was measured by a Pirani-type pressure gaugend the calibration was done using nitrogen. The reactor electrodesonsisted of a cylindrical Pyrex glass tube with a diameter and aength of 60 mm. The energy input frequency was 13.56 MHz. Thehamber was an aluminum made cylinder having a wall thicknessf 2 cm. The useful dimensions of this cylindrical chamber are 200nd 150 mm, for the diameter and the length, respectively.

.3. Morphological analysis

Morphological properties of fibers were determined with aorFi LB-01 fiber analyzer, produced by TECHPAP, France (Passas

t al., 2004). The analysis is done on a fibrous suspension, so thathe measurement occurs in the natural unrestrained environmentf fibers. This approach allows reliable statistical measurement ofhousands of fibers at high speed and accurate determination ofmportant characteristics of their shape. This image analysis wasone for both samples (reference and treated with plasma), afterisintegrating for 20,000 rpm.

.4. Samples characterization

.4.1. Contact angle measurementsA dataphysics OCA (optical contact angle) Absorption Tester

as used in order to select the optimum time for plasma treatmentpplication. In this standard method a droplet of liquid is dispensednto a solid surface and the sessile drop is illuminated from oneide using a diffuse light source and viewed from the other side,n such a way that the contour of the drop is observed. Contactngle measurements were used to characterize the referencend treated samples as described by (Gaiolas et al., 2008, 2009).maller contact angles, formed by a drop of water at the surface ofnvestigated samples, correspond to an increase in its hydrophilicharacter and wettability.

.4.2. Surface characterization by X-ray photoelectronpectroscopy (XPS)

XPS data were obtained using a XR3E2 apparatus (Vacuumenerators, UK) with a mono-chromated MgK� X-ray source

Products 43 (2013) 114– 118 115

(1253.6 eV) and operated at 150 kV under a current of 20 mA. Sam-ples were placed in an ultrahigh vacuum chamber (10−8 mbar) withelectron collection by a hemispherical analyzer at an angle of 90◦.Signal decomposition was done using Spectrum NT and the C 1ssignal was shifted to ensure that the C–H signal of the decompo-sition occurred at 285.0 kV. Spectra were analyzed using Spectrumsoftware.

2.4.3. Image analysisHomogeneity of the distribution of fibers in the hand sheets

was evaluated using a technique based on image analysis by lighttransmission, which consist in taking images of paper samples.The collected images are treated by a statistical approach (first-order entropy), quantifying the homogeneity of the fibrous network(Cresson and Luner, 1990). The first-order entropy is calculatedfrom the array of gray levels in the image. This parameter repre-sents a more or less uniform distribution of the fibers in the planeof the paper sheet. Low values of entropy correspond to a moreuniform paper appearance, which means less contrast of the imagegray levels (Costa, 2001).

3. Results and discussion

3.1. Contact angle results

The first focus of our research was select the optimum timefor plasma application, in order to obtain a paper with the mosthydrophilic character. Therefore several trials were carried outusing different plasma times and the ensuing treated surfaceswere characterized by contact angle measurements. The water con-tact angle for the samples versus the treatment time is shownin Fig. 1.

The treatment of the paper sample by RF-plasma induced adecrease in contact angle of a drop of water from 110◦ to 28◦, fora treatment time of 60 s. Longer treatment times did not yield anyfurther improvement. The rest of experiments were carried out atthis optimum time, because the extension of treatment time didnot induce any significant gain, which does not justify spendingfurther energy consumption, associated with prolonged treatmentduration. The initial paper is surely sized by hydrophobic couplingagents, as commonly practiced is this writing paper grade (Roberts,1996). In fact, lignocellulosic fibers are intrinsically hydrophilicand they usually give low contact angles with water (Gandini andBelgacem, 2011).

0

300250200150100500

Time (s)

Fig. 1. Contact angle versus plasma treatment time, under: 200 W and 700 mTorr.

116 C. Gaiolas et al. / Industrial Crops and

O

O

OH

OH

OH

C1

C6

C5

C4

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TAs

of carbon atoms in these to samples. As mentioned before, this is

C2C3

Scheme 1.

nd the surface can be calculated according to Young–Duprès Eq.1):

TA = �T

L (1 + cos �w/pap) (1)

here WTA is the work of adhesion of the liquid to a solid. �T

L is theurface tension of the liquid (water in our case, i.e., 72.8 mJ/m2 at0 ◦C). �w/pap is the contact angle formed by water on the paperurface.

Before treatment, WTA was close to 48 mJ/m2, whereas after

lasma discharge, WTA is practically three times higher, i.e.,

37 mJ/m2.This result suggests that the occurrence of chemical changes

nduced by plasma treatment on the paper surface; In fact, it isnown that electrical discharge treatments (plasma or corona)ntroduces reactive polar groups and/or increases the surfaceoughness of paper (Belgacem et al., 1995; Cheng et al., 2006; Chent al., 2011). In order to confirm these hypotheses XPS analysesere carried out.

.2. XPS

The most relevant works dealing with the use of X-ray photo-lectron spectroscopy to characterize cellulose substrates wereecently reviewed by Gandini and Belgacem (2011). These workshowed that the low resolution C 1s spectra of cellulose revealedhe presence of two main atoms situated at 285 and 531 eV andttributed to carbon and oxygen atoms, respectively. The decon-olution of C 1s peak shows that three entities are associated witharbon signal and centered at 285.0, 286.7 and 288.3 eV. These moi-ties were attributed to C1 (C–H), C2 (C–O) and C3 (O–C–O and/or

O), respectively.The following summary and scheme can give a useful picture of

PS considerations, as applied to cellulose (Gandini and Belgacem,011). In theory, pure cellulose exhibits two peaks in its deconvo-

uted C 1s XPS spectra, namely: (i) C–O at 286.7 eV and associatedo alcohols and ether groups. This peak is noted as C2 and corre-ponds to 5 carbon atoms (C2–C6 in Scheme 1), and (ii) O–C–O at88.3 attributed to acetal moieties. This signal corresponds to onearbon atom (C1 in Scheme 1).

The surface O/C ratio for pure cellulose (theoretical formula)s 0.83. For the majority of virgin cellulose (avicel, wood pulps,

nnual plants, etc.), this ratio is systematically lower, becausef the presence C-rich molecular segments at the surface of theolids under study (Gandini and Belgacem, 2011). In the presentork the low resolution spectra of the samples, before and after

able 1tomic surface composition of paper samples, as deduced from low resolution XPSpectra.

Paper samples Surface composition (%) O/C

C O

Virgin 60.92 37.04 0.61Plasma-treated 58.04 39.55 0.68Virgin and recycled 60.44 38.63 0.64Plasma-treated and recycled 59.27 39.56 0.67

Products 43 (2013) 114– 118

modification, as well as those prepared from recycled fibersshow that the main peaks detected originate from two mainelements situated at 285 and 531 eV and attributed to carbon(C 1s) and oxygen (O 1s) atoms, respectively. The quantitativedata associated with these spectra are summarized in Table 1.For the untreated sized paper, the O/C ratio is much lower thanthat theoretically calculated (about 0.61 instead of 0.83), whichsuggests that the sizing agent used to make the surface of cellulosehydrophobic is a carbon-rich molecule. In fact, the most commonmolecules used for sizing writing paper are, alkylketene dim-mer, AKD, and alkenylsuccinic anhydride, ASA (see the chemicalstructure below).

AKD ASA

O

R

R'O

OO O

( )n( )m

where R and R′ are C11–C17 aliphatic sequences and m and n havea value of around 10.

The presence of these long aliphatic chains explains the low O/Cratio, since in these molecules the O/C ratio is between 0.05 and0.15.

After plasma treatment, the O/C ratio increased to reach a valueof 0.68, indicating that the sizing agent was oxidized by plasmatreatment, which corroborates the contact angle measurementsstudy. The wide spectra of the recycled virgin samples and theplasma-treated and recycled papers displayed O/C ratios of 0.64and 0.67, respectively.

For virgin samples, the deconvolution of C 1s peak shows thatthe dominant contribution is associated with C1 signals (amount-ing to ca. 26%), attributed to C–C and/or C–H links, as shownin Fig. 2 and summarized in Table 2. These moieties are mostprobably aliphatic and/or aromatic oxygen-free sequences, asso-ciated with the presence of sizing agents. In fact, theoreticallyspeaking, pure cellulose does possess neither C1 nor C4 atoms.In practice, this signal is present, even for cellulose of highpurity, such as microcrystalline cellulose (avicel) (Gandini andBelgacem, 2011).

After plasma treatment, the amounts of C1 decreased drastically(from more than 26 to less than 15%) and the signal associated withC4 increased from around 1 to more than 6%. The evolution of thesetwo signals indicates that the plasma ionized atmosphere reactedwith the paper surface and oxidized the major part of aliphaticsequences. These results agree with those established by contactangle measurements. The C 1s deconvoluted spectra of the recycledinitial paper and the plasma-treated and recycled samples dis-played very close properties. In fact, the intensity of the four peaksis roughly the same for the two recycled samples. This indicatesthat no differences are observed between the content of each type

most probably due to the limited effect of plasma treatment.

Table 2C 1s deconvolution of carbon atoms present at the surface of cotton yarn surface, asdeduced from high resolution XPS spectra.

C 1s deconvoluted surface composition (%)

Carbon type C1 C2 C3 C4

Energy (eV) 285 286.5 288.1 289.8Virgin paper samples 26.8 57.1 14.8 1.3Plasma-treated 14.5 60.5 18.7 6.3Virgin and recycled 25.1 58.9 15.0 1.0Plasma-treated and recycled 21.1 60.3 16.2 2.4

C. Gaiolas et al. / Industrial Crops and Products 43 (2013) 114– 118 117

A

282284286288290292

Binding Energy (eV)

C1s

C-C or C-H

C-O

O-C-O or C=O

O-C=O

B

282284286288290292

C1s

C-C or C-H

C-O

O-C-O or C=O

O-C=O

Fig. 2. C 1s deconvoluted spectra of (A) virgin

Table 3Morphological properties of fibers, before and after plasma treatment.

Plasma treatment

Before After

Length weighted in length (mm) 0.831 0.845Width (�m) 23.8 24.4

3

acolasfitiat

cussed before, this is probably due to the removal and/or oxidation

F

Fines content (% in length) 80.2 79.2Kink angles (◦) 126 127

.3. MorFI analysis

The morphological properties of the fibers were established,s summarized in Table 3. These data shows that no signifi-ant changes in morphological characteristics of the fibers werebserved between the treated and untreated samples. Thus, theength-weighted fiber length was 0.831 mm for the reference fibersnd 0.845 mm for the treated ones. The width of the fiber shows theame tendency, i.e., 23.8 and 24.4 �m, for treated and untreatedbers. The slight increase of the fiber dimensions could result from

heir higher ability to swell because of the oxidation reactionsnduced by plasma treatment. The other parameters (kink anglesnd fine contents) remained practically unchanged, indicating thathey did not suffer any significant damage.

ig. 3. Images of hand sheets of recycled paper, using laboratory disintegrator with differ

Binding Energy (eV)

, and (B) plasma-treated paper samples.

3.4. Image analysis

The reference and plasma-treated samples were disintegratedand the ensuing pulp suspensions were used to produce severalhand sheets. The formation quality (homogeneity) of these sam-ples was evaluated by analyzing the images shown in Fig. 3. Theseimages show that samples arising from plasma-treated fibers seemto have a more homogeneous fibers distribution. Moreover, fibersrecycled in the range of 2500–5000 rpm gave paper with increaseduniformity, as confirmed by quantitative values of entropy.Thus,the image treatment of the obtained photographs gave rise to a wayof quantifying the effect of plasma treatment on the quality of disin-tegrated paper. Fig. 4 illustrates the obtained results and shows thatthe homogeneity of the prepared samples is systematically betterfor the paper made from plasma-treated samples. Now, if one wereto fix a target value of entropy (equal to 5.6 and suitable for writ-ing paper grade), then, this value is obtained after 10,000 rpm foruntreated samples and at 5000 rpm for the plasma treated coun-terpart. Therefore, the discharge treatment permitted performingdisintegration with a substantial energy and time saving. As dis-

of the hydrophobic layers from the paper surface, which enhancethe water penetration into the fiber mat during the recycling andhigher swelling ability of the fibers after plasma treatment.

ent number of rotations. Reference paper (R) and paper treated with plasma (PT).

118 C. Gaiolas et al. / Industrial Crops and

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4

rwprTttfifi

A

fm

R

A

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sion in plasma-treated polyethylene/paper laminates. Int. J. Adhes. Adhes. 7,

ig. 4. Evolution of index of the first order entropy as a function of the numberf rotations applied in disintegration process, for the reference and plasma treatedapers.

. Conclusions

Plasma treatment can be applied usefully in the field of paperecycling and permit reducing the time and the energy associatedith these operations. The surface changes observed in the treatedlasma indicates that the sized initial paper undergoes oxidationeaction, which shifts the surface from hydrophobic to hydrophilic.hus, contact angle measurement and XPS analyses revealed thathe quantity of carboxyl functions significantly increased, whereashat of aliphatic sequences decreased. The plasma-assisted recycledbers did not suffer any morphological damage. They gave recycledber with very good uniformity and fiber distribution homogeneity.

cknowledgements

The authors thank FCT (Fundac ão para a Ciência e Tecnologia)or the awarding of post-doc grant to Carla Gaiolas, within the Com-

unity Support Framework III.

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